How does a surface grinder work.

[SmoggyTurnip]

Hi everyone,

I am very new to this machining stuff.
About 9 months ago I bought a X3 milling machine and a C6 Lathe.
The milling machine is the first one I have ever seen in real life. I am really only starting to learn how to use both of the machines now.


I have spent hours and hours reading this site and I want to CNC the milling machine some day.

I keep reading about surface grinders and I have never heard of such a machine before and I can't understand exactly how they work.

I know that they are used to make very precise surfaces by grind the metal off. What I cant understand is why the grinding wheel does not loose diameter as it grinds thus making the surface uneven as time goes by.



Thanks in advance,
Smoggy

PS this is my first post

[Andre' B]

Originally Posted by SmoggyTurnip What I cant understand is why the grinding wheel does not loose diameter as it grinds thus making the surface uneven as time goes by. Thanks in advance, Smoggy

They do, so every so often you have to dress the wheel, normally done with a diamond tipped tool (but there are other ways) that is feed across the face of the wheel like a lathe turning operation. This reduces the diameter of the wheel but since the operator or the CNC control knows the position of the diamond relative to the part it is possible to adjust for the change in size and go right back to grinding where you stopped.
The diamond also changes size as it is used but that is slow enough not to be a big problem, just need to bump an offset now and then.

How often the wheel has to be dressed depends on the type of wheel, what is being ground, the finish you want, the tolerances you are holding, etc..
How deep and fast the diamond is feed across the wheel has a big affect on how the wheel grinds and the finish left on the part. So often different dressing parameters are used for rough grinding and the finish grinding passes.
Sometimes you even use different grinding wheels for roughing and finishing.

[SmoggyTurnip]

Thanks Andre'b for the quick reply.

So I guess that the wheels on surface grinders are made of a much harder material than the average shop grinder?

[Andre' B]

Actually they tend to be softer.
The cutting material in the wheel is very hard but is basically the same stuff, aluminum oxide.
The bond or the glue that holds the grit together determines the hardness of the wheel.

Softer wheels break down faster in use, which exposes new sharp grains of cutting material. If the wheel does not break down fast enough it gets dull and heats up the part.
Which leads to the not so intuitive fact that the harder the part being ground the softer (the bond) the grinding wheel should be so it cuts cooler and faster. You do have to dress the wheel more often.

Which is why the development of super abrasive grinding is very popular in production, abrasives like diamonds, and cubic boron nitride stay sharp a lot longer. Super abrasive wheels can be very expensive compared to conventional wheels but they will cut faster and last a lot longer unless damaged by operator error.

[ironDigit]

The trick to keep the dressing low as possible is applied :

Picture the wheel turning along with y-axis(thats the one moving from left to right ,wright ?) and the wheel being one inch wide maybe ten inch diameter. The cuttting dept is adjusted by Z just enough to grind a couple a 0,01" of the top ,the workpice is fed into the wheel by the X-axis just a couple o 0.1" and the the y(from left to right i hope) is used to complete the first cycle of flatgrinding a workpiece.If you repeat this cycle over again the load will be evenly devided over grindingstone full-width resulting in minimum wear on the last part of the grindingstone to cut and maximum wear on the first part of the grinding stone wich entered the workpiece in the beginning.

this was discussed an photographed before so a search should make it clear.

[srstol]

Not all surface grinding is done on a machine with the wheel spinning on a horizontal shaft above the table. Rotary surface grinding (blanchard) has the wheel spinning with the axis vertical above the table. To get the surface ground the wheel is lowered onto the part which is spinning on a chuck underneath the wheel. With this style grinder, often referred to as Blanchard grinding because the Blanchard company made the most common rotary surface grinders, the wheel is always loosing media, but never changing diameter.

http://video.google.com/videoplay?docid=5198355931065342085&q=blanchard+grinding&total=15&start=0&num=10&so=0&type=search&plindex=3
This process is great for getting large pieces flat and faces parallel. You can really remove a lot of material in a hurry. At our shop we have an 84" Madison and a 36" Blanchard. They are pretty awesome machines.

Rob

[philserveng]

surface grinding machines are very old technology, they have been around for maybe 100 years. they work on the principal that if you accurately hold the workpiece and skim the material slowly with the grinding wheel, then you can produce a very flat surface. the grinding wheel does wear, but it can be dressed using an industrial diamond to repair any damage. you will then need to advance the wheel a little to account for the loss of wheel diameter. the wheel can reduce in diameter a surprising amount before replacement.